The present invention relates to a heat exchanger applicable, but not limited to, an evaporator of an automotive air conditioning system and, more particularly, to a heat exchanger of the type having fins and a tube one or both of which are made of aluminum or alloy thereof.
An automotive air conditioning system, for example, incorporates two independent heat exchangers in the form of a condenser and an evaporator. The condenser liquefies a high-temperature high-pressure refrigerant by cooling it, while the evaporator cools ambient air by evaporating the liquefied refrigerant. Each of such heat exchangers is constituted by a tube which allows the refrigerant to flow therethrough, and fins held in tight contact with the outer periphery of the tube for radiating the heat of the refrigerant to surrounding air. Generally, the fins and tube or at least the fins are made of aluminum or alloy thereof to provide the heat exchanger with a light-weight construction.
Since the refrigerant flowing through the evaporator lowers the surface temperature of the evaporator due to evaporation heat, moisture contained in an air stream around the evaporator is condensed and apt to deposite on the surfaces of the fins together with dust and other impurities. Water thus deposited on the fins during the operation of the air conditioning system undesirably keeps the surfaces of the evaporator wet over a long period of time even after the deactivation of the system, because the gap between nearby fins is too small to promote drying. Especially, when the air conditioning system is used very often as in summer, the evaporator suffers from temperature and humidity which are easy to glow mold every time the operation of the system is interrupted. The mold causes the dust-containing water deposited on the surfaces of the evaporator to become rotten. As a result, when the system is restarted after some interval, air cooled in contact with the evaporator and introduced into a compartment such as a vehicle cabin often entrains a putrid smell.
Mold of the above discussed nature will be eliminated if an organic high polymer containing fungicide is applied to the surfaces of the evaporator to form a protective layer, as disclosed in Japanese Patent Laid-Open Publication (Kokai) Nos. 58-102073 and 60-50397 by way of example. Another anti-mold implementation may be spraying fungicide onto the surfaces of the evaporator at an adequate time, as shown and described in Japanese Patent Laid-Open Publication No. 59-45213.
The fungicide-containing organic high polymer scheme, however, has a problem regarding the affinity and heat conductivity of the polymer coating and aluminum which forms the evaporator. Hence, the fungicide-containing high polymer needs a delicate and complicated composition, increasing the cost of an evaporator. Moreover, the efficiency attainable with such an implementation is low because it is only the fungicide exposed to the ambience on the coating surface that is effective.
The fungicide-spraying scheme, i.e., spraying fungicide at an adequate time such as at the start of operation of the system is not practicable without needing a special fungicide reservoir and a device for spraying fungicide fed thereto from the reservoir, also resulting in the increase in cost. In the case of an automotive vehicle, exclusive spaces have to be secured in an engine compartment for accommodating the reservoir, spraying device, etc. Nozzles for spraying the fungicide need to be located in front of the evaporator and, therefore, block the stream of air around the evaporator. Further, the amount of fungicide capable of depositing on the surfaces of the evaporator is too small to guarantee the anti-mold effect over a substantial period of time, and the sprayed fungicide is undesirably entrained by cooled air into the compartment.
A heat exchanger made of aluminum is generally subjected to surface treatment which relies on anodization, for the purpose of enhancing corrosion resistance. An anodized layer formed on a heat exchanger by such surface treatment has a number of pores therein. It has been customary to close the pores by sealing, as taught in Japanese Patent Laid-Open Publication No. 60-134198.